The present invention relates to an electrophotographic photoconductor with minimized variations in the electric properties thereof by the ambient conditions, which comprises an electroconductive substrate, an intermediate layer, a charge generating layer and a charge transporting layer, which layers are successively overlaid on the electroconductive substrate.
Conventionally, a variety of methods have been proposed and tried for the purposes of increasing the photosensitivities and minimizing the residual potential of electrophotographic photoconductors comprising a charge generating layer and a charge transporting layer which are successively overlaid on an electroconductive substrate.
For instance, there has been proposed a method of interposing an intermediate layer between the electroconductive substrate and the charge generating layer of an electrophotographic photoconductor for preventing the electric charge having a polarity opposite to that of the electric charge applied to the electrophotographic photoconductor for image formation from being injected from the electroconductive substrate to the charge generating layer during the application of the electric charge for image formation.
Specifically, intermediate layers comprising cellulose nitrate are proposed in Japanese Laid-Open Patent Application Nos. 47-6341, 48-3544 and 48-12034; intermediate layers comprising a nylon resin in Japanese Laid-Open Patent Application Nos. 48-47344, 52-25638, 58-30757, 58-63945, 58-95351, 58-98739 and 60-66258; an intermediate layer comprising a vinyl acetate resin in Japanese Laid-Open Patent Application No. 48-26141; intermediate layers comprising a maleic acid resin in Japanese Laid-Open Patent Application Nos. 49-69332 and 52-10138; and an intermediate layer comprising a polyvinyl alcohol resin in Japanese Laid-Open Patent Application No. 53-100240.
Further in order to control the electric resistivity of such intermediate layers, a variety of electroconductive additives to be contained in the intermediate layers have been proposed. For example, an intermediate layer comprising carbon or a chalcogen type material which is dispersed in a setting type resin is proposed in Japanese Laid-Open Patent Application No. 51-65942; an intermediate layer comprising a polymer which is thermally polymerized using an isocyanate type setting agent with addition thereto of a quaternary ammonium salt; an intermediate layer comprising a resin with addition thereto of an electric resistivity adjustment agent in Japanese Laid-Open Patent Application No. 55-1180451; an intermediate layer comprising a resin in which aluminum oxide or tin oxide is dispersed in Japanese Laid-Open Patent Application No. 58-58556; an intermediate layer comprising a resin in which an organometallic compound is dispersed in Japanese Laid-Open Patent Application No. 58-93062; intermediate layers comprising a resin in which electroconductive particles are dispersed in Japanese Laid-Open Patent Application Nos. 58-93063, 60-97363 and 60-111255; and intermediate layers comprising a resin in which TiO.sub.2 and SnO.sub.2 powders are dispersed in Japanese Laid-Open Patent Application Nos. 59-84257, 59-93453 and 60-32054.
Further, in order to control the charge transfer in the intermediate layer instead of the electric resistivity of the intermediate layer, there have been proposed intermediate layers comprising a resin in which an electron-accepting organic compound serving as a negative charge transporting material is dispersed.
For instance, an intermediate layer comprising an organic photoconductive polymer with addition thereto of a polycyclic aromatic nitro compound is proposed in Japanese Laid-Open Patent Application No. 53-89433; and intermediate layers comprising a resin containing therein an elctron-accepting organic material in Japanese Laid-Open Patent Application Nos. 54-4134, 59-160147 and 59-170846.
Furthermore, Japanese Laid-Open Patent Application No. 59-65852 discloses a photoconductor comprising an electroconductive substrate, a first charge generating layer containing a selenium compound having high photosensitivity to short-wavelength light, a second charge generating layer in which an organic pigment having high photosensitivity to long-wavelength light (such as a phthalocyanine type pigment, a squarylium type pigment and a cyanine type pigment) is dispersed, and an organic charge transporting layer, which layers are successively overlaid on the electroconductive substrate.
However, the first mentioned photoconductors having intermediate layers consisting of only resins are prepared so as to obtain high chargeability and low residual potential by controlling the electric resistivity of the intermediate layers. In those intermediate layers, insulating resins having relatively low resistivities ranging from 10.sup.10 .OMEGA.cm to 10.sup.14 .OMEGA.cm are employed. In order to increase the chargeability of the photoconductors, it is necessary to increase the thickness of the intermediate layers, while in order to reduce the residual potential of the photoconductors after exposure to light, it is necessary to decrease the thickness of the intermediate layers. Therefore it is extremely difficult to obtain both high chargeability and low residual potential at the same time by using these intermediate layers. Further, these intermediate layers are susceptible to the water contained in the air, so that at low temperatures and high humidities, the photoconductors having these intermediate layers tend to have high chargeability and high residual potential, while at high temperatures and high humidities, they tend to have low chargeability and low residual potential. In short, those photoconductors have the shortcoming that their electric properties are considerably susceptible to the changes in the ambient conditions.
In the photoconductors having an intermediate layer of the type comprising a resin and an electroconductive additive, the electric resistivity of the intermedate layer is controlled by the electroconductive additive. However, as in the case of the intermediate layer consisting of only a resin, the electric resistivity of the intermediate layer has a significant effect on the electric characteristics of the photoconductor. This intermediate layer has the same problem as in the case of the intermediate layer consisting of only a resin. Furthermore, the electroconductive additive contained is susceptible to the water contained in the air, so that it is still very difficult to meet the requirements for high chargeability and low residual potential by use of such intermediate layer.
In the photoconductors having an intermediate layer comprising a resin and an electron-accepting organic compound, the electric characteristics thereof are not so susceptible to the changes in the ambient conditions as in the above-mentioned photoconductors. However, such electron-accepting organic compounds are highly soluble in organic solvents. Therefore, the electron-accepting organic materials are apt to mix with the charge generating layer and the charge transporting layer during the coating of these layers and even during the drying thereof. When such mixing takes place, the light decay photosensitivity of the photoconductors is significantly decreased. In addition, such electron-accepting organic materials are apt to be highly crystallized and have poor compatibility with the resins to be used in combination, so that such crystallization may take place during the preparation of the photoconductors.
The previously mentioned photoconductors having an intermediate layer comprising a selenium compound and an organic pigment have the shortcoming that the process of their production is extremely complex.